In a multimode optical fiber, various transmission mode components propagate with different group velocities, this effect causes the delay distortion in multimode optical fiber transmission systems. As is known, the energy radiated from the end of the fiber is concentrated along a plurality of cones, where each mode has a characteristic cone angle. Thus, by separately detecting the different modes, or groups of modes according to the radiation pattern, it becomes possible to delay the resultant output signals by an appropriate amount relative to each other so as to compensate for the dispersion introduced in the optical transmission wavepath. The signals are then combined in a common output circuit.
As a means for separating the radiation pattern by the wave energy emitted by the fiber, a composite lens has been proposed by Y. Suematsu et al. in "Refractive Index Distribution and Group Delay Characteristics in Multimode Dielectric Optical Waveguides" p. 111 (FIG. 9) published in Electronics and Communications in Japan, Vol. 57-C, No. 9, 1974, pages 105 - 112. According to this technique, lower-order and higher-order mode rays incident on the center and the peripheral areas, respectively, of the composite lens are converged at different positions on the same axis. Input ends of optical fibers different in length from each other which function as delay means are located at the individual light-converging positions. In this construction, however, the lower-order mode rays are converged into the inside of the converging cone formed by the higher-order mode rays. This results in a part of the higher-order mode ray being intercepted by the optical fiber support member used to couple the lower-order mode ray to the optical fiber and by the optical fiber itself. This is why optical loss has been large in the prior art mode separator.
One solution to this problem is found in S. E. Miller's U.S. Pat. No. 3,777,150 issued Dec. 4, 1973. In the Miller Invention, a concentric, circular photodetector for mode separation is located directly near the output end of the multimode optical fiber. One of the problems associated with such system is that the construction of the photodectector is intricate, resulting in low productivity.
It is therefore an object of the invention to provide a mode separator for use in optical fiber communication systems, which can be manufactured with ease and operated with a minimum of optical loss.
With this and other objects in view, the invention provides a mode separator comprising an array of concentric, circular, light converging regions, which is located adjacent to the optical fiber end in a plane perpendicular to the fiber axis. Lower-order mode light rays are converged in the center portions of the individual light converging regions; and higher order mode light rays, in the peripheral portions thereof. The focal lengths and the optical axes of the individual light converging regions are so determined that each of converging points is located outside of other cone-shaped converging mode rays formed by other light converging regions. Thus, by employing eccentric optical systems having different optical axes in the individual light converging regions, a mode separator for optical fiber communications, operable with low optical loss and manufacturable at low costs can be realized.